The technology of plasma display panels (PDPs) allows large flat display screens to be produced. PDPs generally comprise two insulating plates defining between them a gas-filled space in which elementary spaces bounded by barrier ribs are defined. One of the two plates is provided with an array of row electrodes and the other is provided with an array of column electrodes. An elementary cell corresponds to an elementary space provided with at least a row electrode and a column electrode that are placed on either side of the said elementary space. To activate an elementary cell, an electrical discharge is generated in the corresponding elementary space by applying a voltage between the row and column electrodes of the cell. The electrical discharge then causes the emission of UV radiation in the elementary cell. Phosphors deposited on the walls of the cell convert the UV into visible light. The cell will be red, green or blue depending on the nature of the phosphor deposited on its walls.
Unlike cathode-ray tube or liquid-crystal screens in which the video levels are obtained by modulating the amplitude of the voltage signal applied to the electrodes of the cell, a PDP controls the video levels by modulating the duration of ignition or the on time of the cells during a video frame, that is to say the gas contained in the cell is excited for a longer or shorter time depending on the desired grey level. The human eye then performs a time integration in order to recreate the grey level.
Consequently, the cells of the PDP have only two states: the on (excited) state or the off (unexcited) state. The cell is maintained in one of these states by the sending of a succession of pulses called sustain pulses over the desired duration of ignition. The cell is addressed by the sending of a higher electrical pulse, usually called an address pulse. Extinction, or erasure, of the cell is accomplished by eliminating the charges inside the cell using a damped discharge.
The various grey levels are obtained by modulating the duration of the successive on and off states of the cell over the course of the video frame. The frame is divided into periods called subfields during each of which the cell may either be on or off. The human eye integrates the periods of illumination of the cell in order to recreate the desired grey level.
FIG. 1 shows a conventional organization of the subfields within the video frame. The duration T of the video frame is 16.6 or 20 ms depending on the country. A minimum of eight subfields, denoted SF1 to SF8, is provided in the frame in order to display an image with 256 possible grey levels. Each of the subfields is used to turn on, or not, the cell for an illumination period of duration Til that is a multiple of an elementary duration T0. Each subfield comprises for this purpose an address period of duration Tad and an illumination period (hatched in the figure) of specific duration Til. The duration Tad is identical for all the subfields and is equal to Nl×Tae, where Nl is the number of lines in an image and Tae is the line address time. On the other hand, the duration Til is specific to each subfield and is equal to p×T0 where p is an integer denoting the weight of the subfield in question. In the example shown in FIG. 1, the subfields SF1, SF2, SF3, SF4, SF5, SF6, SF7 and SF8 have 1, 2, 4, 8, 16, 32, 64 and 128 as respective weights. Thus, the video level of each colour component (R or G or B) will be represented by an 8-bit word, each bit being associated with one subfield of the frame. Of course, other organizations of subfields having a larger number of subfields or subfields with different weights may be employed.
Although this PDP technology offers the possibility of producing large screens of small thickness, it does have, however, drawbacks that degrade the quality of the image displayed. These drawbacks are associated with the time integration of the illumination periods over the course of the video frame. A problem of false contouring appears, especially when a point on the screen moves during several consecutive images. This defect is manifested in the image by the appearance of darker or lighter bands at grey level transitions that normally are barely perceptible.
This false contouring problem is illustrated in FIG. 2 which shows the subfields for two consecutive frames, F and F+1, having a transition between a 127 grey level and a 128 grey level. This transition moves by four pixels between the two frames. In the figure, the y-axis represents the time axis and the x-axis represents the pixels of the images that are displayed during the said frames. Integration carried out by the eye amounts to integrating over time along the oblique lines shown in the figure, as the eye has a tendency to follow the transition that moves. The eye therefore integrates the information coming from different pixels. The result of the integration gives the appearance of a grey level equal to zero at the moment of transition between the 127 and 128 grey levels. This passing through the zero grey level produces a dark band at the transition. In the reverse case, if the transition passes from the 128 level to the 127 level, a 255 level corresponding to a light band appears at the moment of the transition.
A first known solution for correcting this defect consists in “breaking” the high weights of the subfields in order to reduce the integration error. FIG. 3 shows the same transition as in FIG. 2, but with seven subfields of weight 32 instead of three subfields of weights 32, 64 and 128. The integration error is then at most a grey level value of 32. It is also possible to distribute the grey levels differently, however there still remains an integration error.
In European Patent Application No. 0 978 817, the false contouring effects are compensated for by using a movement estimator that determines movement vectors for blocks of pixels of the image. These movement vectors are used to modify the data delivered to the elementary cells of the PDP. The basic idea of that patent application is to detect the movements of the eye during the display of the images and to deliver movement-compensated data to the cells so that the eye integrates the correct information. This method is illustrated in FIG. 4. Such a correction amounts to displacing the subfields spatially according to the observed movements between the images so as to anticipate the integration that the human eye will perform. The subfields are displaced differently according to their weight and to their temporal position in the video frame. This solution requires a movement estimator that calculates a movement vector for each pixel or each block of pixels of the image. For each pixel, the corresponding movement vector is used to shift the associated code word in the direction of the movement vector. The code words for the pixels of the image are therefore recomputed. This solution gives good results at the transitions that cause false contouring effects but does require the implementation of a movement estimator having a high computing speed. This estimator is relatively expensive and not very easy to produce.
Another solution for compensating for the false contouring effects is based on a novel type of coding called “incremental coding”. This method of coding is described for example in European Patent Application EP-A 952 569. In this method, only a small number of code words are used to display the image on the screen. The codes used have the feature of not including an “off” (respectively “on”) subfield between two “on” (respectively “off”) subfields. This feature makes it possible to completely eliminate the false contouring effects, but it does greatly limit, however, the number of codes that can be used (n+1 possible codes for a frame with n subfields). The grey levels corresponding to the other codes (that cannot be used) are reconstructed on the screen by error diffusion or “dithering” techniques well known to those skilled in the art. The major drawback of this coding is the small number of grey levels that can be displayed on the screen, the dithering techniques not always allowing the lost grey levels of the image to be restored.
Finally, there is a last solution, also employing a novel coding and introducing less dithering noise. This solution is described in the European Patent Application filed on 8 May 2001, the filing number of which is 01250158.1. This novel coding consists in selecting m video levels from the p video levels that can be displayed with a frame structure having n subfields, where n<m<p. The m video levels are selected so that the temporal centre of gravity of the illumination generated by their code words increases continuously with the video levels, except for the low video levels down to a first predefined limit value and/or for the high video levels from a second predefined limit value. This means that, for two levels GL1 and GL2 that belong to the m selected levels such that GL1>GL2, then the temporal centre of gravity of the code word associated with the level GL1 is higher than that of the code word associated with the level GL2.
The temporal centre of gravity of the illumination generated by a code word is calculated from the following formula:
      CG    ⁡          (      code      )        =                    ∑                  i          =          1                n            ⁢                        W          ⁡                      (                          S              i                        )                          ·                              d            i                    ⁡                      (            code            )                          ·                  CG          ⁡                      (                          SF              i                        )                                              ∑                  i          =          1                n            ⁢                        W          ⁡                      (                          SF              i                        )                          ·                              d            i                    ⁡                      (            code            )                              where:                CG(code) is the centre of gravity of the code word in question;        W(SFi) denotes the weight of the ith subfield (SFi) of the frame;        di(code) is equal to 1 if the ith subfield is on for the code in question, and 0 otherwise; and        CG(SFi) is the centre of gravity of the ith subfield.        
The centre of gravity of the ith subfield, CG(SFi), is calculated in the following manner:CG(SFi)=D(SFi)+Dur(SFi)/2where:                D(SFi) is the time start point of the ith subfield; and        Dur(SFi) is the duration of the ith subfield.        
With this coding, which hereafter will be called GCC (Gravity Centre Coding), the curve showing the centres of gravity of the codes selected as a function of the video levels is monotonic, at the very least between the said first and second predefined limit values, thereby making it possible to eliminate the false contouring effects. Moreover, the number of video levels that can be displayed with this coding is larger than with an incremental coding, thereby allowing the dithering noise to be reduced.
The GCC coding is illustrated in FIGS. 5 to 7. FIG. 5 shows the temporal centres of gravity of all the video words that are possible with a frame structure comprising eleven subfields, the weights of which are as follows:1-2-4-7-11-16-23-32-43-56-60.The y-axis represents the centre-of-gravity value and the x-axis represents the video level of the code word. Since there are eleven subfields, there are 211, i.e. 2048, possible code combinations for the 256 video levels. Corresponding to each video level is therefore one or more code words and therefore one or more centres of gravity. The centre of gravity is calculated from the formulae indicated above. For this calculation, an overall time of 1 ms for addressing and erasing each subfield and a maximum illumination time Tmax of 5.10 ms (corresponding to the sum of the illumination periods of all the subfields of the frame) were considered, which gives an illumination time of 0.02 ms for the subfield of weight 1, an illumination time of 0.04 ms for the subfield of weight 2, . . . , and an illumination time of 1.2 ms for the subfield of weight 60. The corresponding frame then has a duration of 16.1 ms, which corresponds to a frequency of 60 Hz.
FIG. 6 shows the lowest centre-of-gravity value for each video level. This is because it is general to use, in order to code a video level, the video word having the lowest centre of gravity, as it is this one that introduces the fewest false contouring effects because the subfields of lower weight are used. As may be seen, the curve defined by these values is not monotonic, rather it has jumps that inevitably introduce false contouring effects.
GCC coding aims to eliminate these false contouring effects by selecting only a restricted number of video levels, as shown in FIG. 7, so as to obtain a monotonic centre-of-gravity curve. The video levels selected are identified in the figure by a small black diamond.
As may be seen in this figure, the number of levels that meet the GCC coding may be relatively small. The number of video levels selected is therefore small and means that there is always dithering noise when displaying a video image.
The main object of the invention is to alleviate the aforementioned drawback.